bpf: clarify when bpf_trace_printk discards lines

I opened /sys/kernel/tracing/trace once and kept reading from it.
bpf_trace_printk somehow did not seem to work, no entries were appended
to that trace file. It turns out that tracing is disabled when that file
is open. Save the next person some time and document this.

The trace file is described in Documentation/trace/ftrace.rst, however
the implication "tracing is disabled" did not immediate translate to
"bpf_trace_printk silently discards entries".
Signed-off-by: NPeter Wu <peter@lekensteyn.nl>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -580,6 +580,8 @@ union bpf_attr {
  * 		limited to five).
  *
  * 		Each time the helper is called, it appends a line to the trace.
+ * 		Lines are discarded while *\/sys/kernel/debug/tracing/trace* is
+ * 		open, use *\/sys/kernel/debug/tracing/trace_pipe* to avoid this.
  * 		The format of the trace is customizable, and the exact output
  * 		one will get depends on the options set in
  * 		*\/sys/kernel/debug/tracing/trace_options* (see also the